Hey, good on you for still getting after it! A basketweave pattern can be a bit tricky, so when dealing with different sizes, the key is sorting your bricks beforehand. You'll want to lay out your pattern using bricks of the same size within each "square" of the weave. This method helps maintain a consistent, tight-looking pattern. If the size differences are small, like a quarter-inch, you can hide it in the mortar joints without it being noticeable.
To keep your pattern straight and square, use chalk lines. Snap a main control line down the middle of your work area, then use a square to snap perpendicular lines every few feet. This will give you a grid that helps you keep the pattern tight and uniform as you work. You're right that you'll need to be flexible with your mortar joints. It's good to aim for a joint thickness between 3/8 and 1/2 inch. Going much wider than that can compromise the strength of the bond and won't look as clean. As for the mix, a standard ratio is 3 parts sand to 1 part Portland cement. If you're using a premixed mortar, just follow the manufacturer's instructions. Good luck with the project!